辽－吉古元古代孔兹岩系的变质演化与成因

There is controversy on tectonic setting of the Jiao-Liao-Ji Belt in the north China craton for many years. The key problem is that the metamorphism of the Liao-Ji khondolite series is still unclear, which exists both clock-wise and anticlock-wise PT path. Using the software THERMOCALC, the proposal applicant will calculate P-T pseudosections for typical AL-rich gneiss from south Liaohe group (including Ji'an group) and north Liaohe group (including Laoling group). In these phase diagram, the metamorphic peak PT conditions and PT path could be determined by comparing actual compositons of mineral with calculated results. Moreover, by calculating P-T pseudosections in the system with TiO2, content of zircon produced during metamorphism could be showed in the diagram, which is very important for determining implications of zircon U-Pb and Lu-Hf dating results. As a result, metamorphic age and process of Liao-Ji khondolite series will be derived, which could provide important constraints on tectonic evolution of the Jiao-Liao-Ji Belt.

胶-辽-吉古元古代活动带的大地构造属性问题争议多年，关键是对北部辽-吉孔兹岩系的变质演化认识不清，出现顺时针与逆时针PT轨迹共存的现象。通过THERMOCALC程序定量计算典型变质岩石NCKFMASHTO体系的P-T视剖面图，重新构筑南辽河群（含集安群）与北辽河群（含老岭群）的变质作用PT演化轨迹，查明逆向PT轨迹出现的原因。通过含ZrO2体系的P-T视剖面图，分析变质锆石的演化过程，明确锆石U-Pb定年和Lu-Hf同位素分析结果的指示意义。确定辽-吉古元古代孔兹岩系的变质演化与多阶段变质时代，为正确认识胶-辽-吉活动带的构造属性提供必要制约。

本项目在详细的野外工作基础上，对华北克拉通胶-辽-吉活动带北部的集安群、光华群和南部的五河群变质岩系开展了系统的岩石学与同位素年代学工作。在通化地区集安群变质表壳岩中发现了含有紫苏辉石+堇青石+条纹长石+尖晶石+石英特征组合的超高温麻粒岩，峰期变质温度在900℃以上，具有顺时针样式的PT演化轨迹，表明该地区曾经历特殊的构造环境，其成因很可能与热造山作用有关。根据锆石U-Pb和Hf同位素分析结果，确定集安群变质表壳岩的沉积时代在2.3Ga以后，变质时代存在1.95～1.90Ga和1.90～1.85Ga两个阶段，可能分别代表变质峰期阶段和峰期后降温冷却阶段。通化地区光华群碎屑锆石年龄都在2.5Ga以上，表明其为太古代变质表壳岩。其部分变质岩石记录了2525±10Ma、1926±40 Ma和1878±16 Ma的变质锆石年龄,表明其既经历了太古代末期的构造热事件改造,又遭受了古元古代晚期碰撞造山事件的扰动。确定侵入光华岩群底部的钾长花岗岩的侵位年龄为2154±7Ma。通化地区TTG片麻岩主要形成于新太古代，Hf模式年龄集中在集中在2.7-2.8Ga，εHf(ｔ)全部为正值，表明为新生陆壳再重熔的产物。安徽蚌埠地区五河群下段（西堌堆组）沉积年龄可达2.7Ga以上，属于太古代，中上段沉积年龄在2.1Ga以上，属于古元古代。